Distillation Flashcards
Why must the thermometer bulb be below the sidearm on the simple distillation apparatus? Can the bulb touch the side of the glass of apparatus? Why?
- So it accurately records the temperature of vapor distilling through the sidearm.
- No, the bulb cannot touch the side of the glass of apparatus because it would take the temperature of the glass, not the vapor
Why must the end of the sidearm be inside the receiver vial?
- To minimize vapors from escaping out results in a loss of liquid.
How do you control the heating of the apparatus?
- Do this by using a spatula to scrape the sand both around or away from the flask.
Why do you use a boiling chip? Why can you only use the boiling chip once?
- Boiling chips prevent superheating.
- Boiling chips cannot be reused because the pores would fill with liquid.
How do boiling chips prevent superheating?
- Boiling chips provide nucleation sites which allow the liquid to boil smoothly.
Why do you use a wet paper towel to cover the condenser (sidearm)?
- To ensure vapor condenses within the sidearm.
Why do you wrap the apparatus is tin foil?
- To insulate and minimize heat loss.
What are the two characteristics of simple distillation?
- ) It is not a complete separation.
2. ) Both liquids are purer than before but are not completely pure.
What is the best criteria for deciding to use simple distillation?
- ) When the boiling point difference between the two liquids is large ( more than 75 degrees C)
- ) When the composition of one liquid is less than 10%
- ) When the liquid has a non-volatile component (like a solid contaminant).
What are the best criteria for fractional distillation?
- ) Boiling point differences between the two liquids is small.
- ) The composition of the two liquids is substantial.
What is fractional distillation?
- It is a series of continuous distillations.
- After a fraction vaporizes, some compound condenses and gets redistilled.
Why do you need to supply more heat to the fractional distillation apparatus vs. the simple distillation apparatus?
- Because the vapor has to travel a farther distance before reaching the condenser in fractional distillation.
What are the differences in apparatus’ between fractional and simple distillation?
- Fractional distillation apparatus is longer and has a copper sponge inside its distilling column.
What are the 5 reasons why a copper sponge is packed in the distilling column of the fractional distillation apparatus?
- ) Easy material to pack within
- ) Does not come out of the column-like beads do
- ) Great heat transfer characteristics
- ) Low hold up
- ) Has a large surface area.
What is hold up?
- Unrecoverable distillate that wets the column packing.
Why must the column be vertical in fractional distillation?
- So the condensing liquid can percolate through the rising hot vapors.
What happens if the upper portion of the column is at a consistently higher temperature than the lower portion of the column?
- The liquid condensate will vaporize as it moves down the column.
What are the 4 important requirements to perform successful fractional distillation and describe them?
- ) Significant contact between the liquid and vapor phases in the column.
- Achieve this by packing the column with something that has a large surface area. - ) Maintenance of the proper temperature gradient along the column
- This determines the rate of distillation so don’t distill too fast. - ) Sufficient length of the column
- Figured out by trial and error - ) Sufficient difference in boiling points between the two liquids
- Should be about 20-30 degree difference.
- If a smaller difference then the length of the column and/or type and amount of packing material must be adjusted.
What happens if you distill too slow?
- A column will get flooded which is characterized by a column of liquid observed in the fractional column at the joint between column and reaction flask.
- Make sure the rate of distillate is adjusted.
Why must the rate of distillation be slow?
- ) So the ascending vapors and descending liquids are in equilibrium.
- ) To maintain a proper temperature gradient in the column.
- ) Overheating can lead to bumping of liquid in the flask.
- ) Underheating can lead to reflux (when vapors condense and return to distilling flask instead of distilling).
What does the column need to be to ensure proper heating? And what is it?
- Needs to be adiabatic (heat doesn’t leave or add to the system)
- Insulate apparatus with foil.
What are the 3 precautions of distilling? Describe them.
- ) Never distill in air tight system (closed system)
- Apparatus may rupture. - ) Never distill to dryness.
- Dry liquid residue can be explosive. - ) Do not fill flask more than 2/3.
- This may cause the liquid to pump and contaminate distillate.
What are 3 sources of error?
- ) Loose connectors
- Will cause vapor to escape. - ) Temperature variations
- Due to airflow in hoods - ) Large surface area of microscale apparatus.
- May loose heat to the environment.
What are the 4 reasons why you may not recover all the liquid to be purified/separated?
- ) The positioning of the sidearm.
- ) Loose connectors and adapters
- ) Hold up in fractional distillation
- ) A significant amount of distillate left adhering to the glass surface of the apparatus.
How do you calibrate the thermometer specifically for carrying out distillation or checking boiling points?
- You check the 0 degrees C point using crushed ice and distilled water.
- You also check the 100 degrees C point using boiling water.
What is the use of distillation?
- To purify liquids using boiling points.
What must the 2 liquids be in order for fractional and simple distillation to work?
- They must be a homogenous layer, miscible in one another.
When do liquids boil?
- When the vapor pressure of liquids=atmospheric pressure as the liquids are heated.
- Higher altitude = lower pressure = lower boiling point
Why doesn’t all liquid evaporate when reaching the boiling point?
- Need to account for the heat of vaporization.
Why are boiling points not as good of indicators of purity as melting points are?
- This is due to vapor pressures which measure how easily molecules escape the surface of liquids.
- Liquids with higher vapor pressures have lower boiling points when compared with liquids with lower vapor pressures.
What are the two characteristics when distilling a pure liquid?
- ) The boiling point of liquid=vapor temperature.
2. ) The temperature remains constant throughout distillation.
What are the characteristics of distilling mixtures?
- ) The temperature does not remain constant; it increases throughout distillation because:
(a. ) the composition of vapors that is distilling varies throughout.
(b. ) some lower boiling point component comes off first then the vapor is enriched with higher boiling point component.
In mixtures what would happen if the sum of the partial pressures = the prevailing atmospheric pressure?
- The liquid would boil
What happens if the packing material in fractional distillation is too dense?
- The column would have pressure changes which would cause nonequilibrium conditions.
What happens if the packing material has too large a surface area?
- It can hold too much of the material being distilled and also cause pressure changes causing nonequilibrium conditions.
What is HETP? And what is its relationship to the efficiency of the column?
- Height equivalent to a theoretical plate is each successive condensation-vaporization cycle, also called a theoretical plate.
- The smaller the HETP, the more plates the column will have and the more efficient it will be.
What are azeotropes?
- Mixtures with the fixed composition that cannot be altered by simple or fractional distillation.
What are the characteristics of azeotropes?
- ) Will act as a pure compound even though it is a mixture.
- ) B.P will remain constant throughout distillation.
- ) The composition of the liquid = composition of vapor.
- ) On the boiling point composition graph, the liquid and vapor curves pinch together at the azeotropes composition.
What are the two types of azeotropes?
- ) Can be minimum boiling (below the pure boiling point of both liquids.
- ) Can be maximum boiling (above the pure boiling point of both liquids.
What are two methods to form 100% ethanol from 95% ethanol? What type of azeotrope is this?
- ) The water can be removed chemically (by a reaction with calcium oxide).
- ) Water can be removed as an azeotrope by adding benzene and distilling it.
- This is a minimum boiling azeotrope.
What is vacuum distilation? When is it used and for what compunds is it used?
- Distillation carried out under reduced pressure.
- Used when ordinary or steam distillation are not practical.
- Used for compounds that decompose at boiling points or sensitive oxidation.
- Liquids must be miscible.
In vacuum distillation why must glassware be checked for scratches or cracks?
- This may cause implosion under the vacuum.
When is steam distillation used?
- Usually with immiscible liquids (one of them being water)
What is a characteristic of steam distillation?
- The boiling point of the mixture is below the boiling point of each pure compound.
What are the advantages of steam distillation?
- ) Volatizing liquids and solids below their boiling points by passing steam into a boiling flask.
- ) This is good for high boiling substances that decompose before boiling point is reached.
What requirement is necessary so that the boiling point remains constant steam distillation?
- Adequate amounts of both water and organic component are present to saturate vapor space
When boiling sugar and water what is the temperature in the vapor phase vs. the temperature in the sugar solution itself?
- The vapor phase is at 100 degrees C, while the sugar solution itself is above 100 degrees C and continues to rise as the concentration of sugar in the remaining solution increases.
What is the vapor pressure of the sugar solution dependent on?
- The number of water molecules present in a given volume.
- With increasing nonvolatile sugar molecules and decreasing concentration of water, the vapor pressure at the given temperature decreases and a higher temperature for boiling increases.
- Sugar molecules do not leave the solution and the drop clinging to the thermometer is pure water in equilibrium with pure water vapor.
Does boiling point change with pressure?
Yes.
What actions do you take to specifically heat a test tube?
- ) A boiling stick is easier to remove than a boiling chip but use a boiling chip instead of a boiling stick in a reaction.
- ) You can hold the test tube in your hand because it is long and narrow.
- ) The sand is hotter deeper down and cool on top providing a range of temperatures.
Why do you need to dilute the solution in filtration in a Pasteur pipette? And how does this process work?
- So the solute will not recrystallize at about 22 degrees C.
1. ) Place cotton in the tip of the pipette and use another pipette to put the solution within this pipette.
2. ) Use the pipette to filter out the solution and use a fresh solvent to rinse the pipette and cotton.
3. ) The filtered solution is then concentrated by evaporation.
What does a change in boiling point during distillation mean for a pure liquid?
- Means there is an impurity present.
Does a constant boiling point indicate only one compind is present?
- No.
1. ) You could have two miscible liquids that have similar boiling points. OR
2. ) Azeotropes mixtures have constant temperatures that are below or above the boiling points of the individual components.
How does structure affect boiling point? (3)
- ) The strength of the intermolecular forces
- Stronger intermolecular forces the lower the boiling point.
- Ionic > Hydrogen > Dipole-Dipole > VDW - ) Boiling point increases with molecular weight.
- ) Branching decreases boiling point
How does structure affect boiling point? (3)
- ) The strength of the intermolecular forces
- Stronger intermolecular forces the lower the boiling point.
- Ionic > Hydrogen > Dipole-Dipole > VDW - ) Boiling point increases with molecular weight.
- ) Branching decreases boiling point
How does atmospheric pressure affect boiling point?
- The boiling point is reached when the vapor pressure of liquid = atmospheric pressure.
- If atmospheric pressure increases, boiling point increase.
- If atmospheric pressure decrease, boiling point decreases.
